(1) Field of the Invention
The present invention relates to metal halide lamps, and in particular relates to a metal halide lamp that uses an arc tube made of alumina ceramic.
(2) Related Art
In recent years, arc tubes made of alumina ceramic have become mainstream ones for use in metal halide lamps, replacing conventional silica glass arc tubes. Alumina ceramic has higher heat-proof properties than silica glass, and therefore is a suitable material for arc tubes used in such high-pressure discharge lamps as metal halide lamps, which reach high temperatures during lighting.
The temperature of such a metal halide lamp using an alumina ceramic arc tube can be raised to high during lighting of the lamp. Therefore, the lamp is enabled to exhibit higher color rendition and higher luminous efficiency.
Further, alumina ceramic has a lower reactivity to metal halide enclosed in an arc tube than silica glass. With the use of such alumina ceramic arc tubes, therefore, metal halide lamps are expected to have a longer life.
Here, a method for sealing electrodes in this type of lamp is different from a sealing method employed for a lamp using a silica glass arc tube. The sealing method employed for the silica glass arc tube is to seal electrodes by heating and crushing ends of side-tube parts of the arc tube. Unlike this method, the sealing method employed for the alumina ceramic arc tube is to first insert a power feeding member into a space formed within each of two thin-tube parts, and then inject a melted sealing material, e.g., a glass frit, into the space, thereby sealing the power feeding member in each thin-tube part. According to this method, however, the power feeding member and the thin-tube part are sealed only at the end of each thin-tube part not facing the discharge space via the sealing material, and the unsealed area results in a gap between the power feeding member and the thin-tube part (see e.g., Japanese Laid-open Patent Application No. S57-78763). Such a gap is inevitably large for a large-size lamp with a high wattage.
As described above, a conventional metal halide lamp that uses an alumina ceramic arc tube has gaps between its power feeding members and thin-tube parts of the arc tube. When this conventional lamp is lit with the electrodes being oriented in the vertical direction, a light-emitting metal enclosed within the arc tube is likely to flow into the gap in the lower one of the thin-tube parts in the vertical direction.
If a portion of the light-emitting metal flows into the gap during a test-life of the lamp (hereafter simply a xe2x80x9clifexe2x80x9d) the amount of metal contributing to light emission in the discharge space decreases accordingly. If this happens, a sufficiently high vapor pressure cannot be obtained. The lamp then suffers from the problem that its color temperature changes greatly as the lamp is lit for long hours.
To solve such problems, Japanese Laid-open Patent Application No. 2000-340171 discloses the lamp construction where, in each thin-tube part, at least a predetermined distance is provided between (a) one end of the thin-tube part facing the discharge space and (b) the electrode coil.
FIG. 1 shows a metal halide lamp with the conventional construction according to the disclosure. This metal halide lamp is made up of a light-emitting unit 102, thin-tube units 103a and 103b, a pair of electrodes 105a and 105b, electrode holding members 106a and 106b, and sealing members 107a and 107b. The light-emitting unit 102 is made of translucent ceramic and in which a discharge space 101 is formed. In the discharge space 101, a light-emitting metal is enclosed. The thin-tube units 103a and 103b are respectively provided at both ends of the light-emitting unit 102. The pair of electrodes 105a and 105b respectively have coils 104a and 104b at their tops. The electrode holding members 106a and 106b respectively hold the electrodes 105a and 105b at their one ends. The other ends of the electrode holding members 106a and 106b extend from the ends of the thin-tube units 103a and 103b not facing the discharge space 101. The sealing members 107a and 107b are respectively provided to seal the electrode holding members 106a and 106b to the thin-tube units 103a and 103b. According to the disclosure, the metal halide lamp is to be constructed to satisfy the condition xe2x80x9cX greater than 0.0056P+0.394xe2x80x9d, where xe2x80x9cP [W]xe2x80x9d represents a lamp wattage, and xe2x80x9cX [mm]xe2x80x9d represents a distance from one end of the coil 104a (104b) facing the thin-tube unit 103a (103b) to one end of the thin-tube unit 103a (103b) facing the discharge space 1.
By satisfying this condition, the temperature of the ends of the thin-tube units 103a and 103b facing the discharge space can be lowered to such a degree that an excess of light-emitting metal enclosed in the discharge space can exist in a liquid form therein. Therefore, the amount of light-emitting metal flowing into the gaps in each thin-tube unit can be reduced, thereby reducing the color temperature change.
According to the above disclosure, however, means for occupying the gap xe2x80x9cxcexxe2x80x9d formed within each of the thin-tube units 103a and 103b is not provided. Even if a certain conventional technique provides a member as this means for occupying the gap xe2x80x9cxcexxe2x80x9d, the member is entirely embedded in a thin-tube unit so as to be recessed from the end of the thin-tube unit, and fails to prevent a light-emitting metal from easily flowing info the gap xe2x80x9cxcexxe2x80x9d, thereby failing to prevent the color temperature from being greatly changed after continuous lighting of long hours.
The present invention therefore aims at providing a metal halide lamp that exhibits stable characteristics with a reduced change in the color temperature even after continuous lighting of long hours, by reducing the amount of light-emitting metal flowing into gaps formed in thin-tube units of an arc tube.
The above aim can be achieved by a metal halide lamp, including: a bulb that is made up of a light-emitting unit in which a discharge space is formed, and a pair of thin-tube units each being fitted into an opening in a different one of both ends of the light-emitting unit; a pair of electrodes that extend into the discharge space so that tops thereof are opposed to each other, each electrode having an electrode coil at a top part thereof; a pair of electrode holding members each being provided through a different one of the thin-tube units so that one end thereof holds the electrode and the other end thereof extends from one end of the thin-tube unit not facing the discharge space, each electrode holding member being sealed to the thin-tube unit via a sealing member; a tubular member that is provided in at least one of the thin-tube units, so that the electrode and/or the electrode holding member therein is inserted through the tubular member, the tubular member being made of a heat-proof and heat-conductive material, wherein the expressions xe2x80x9cXxe2x89xa70.0056P+0.194xe2x80x9d and xe2x80x9c0 xe2x89xa6Lxe2x89xa60.44Xxe2x80x9d are satisfied, where xe2x80x9cPxe2x80x9d is a lamp wattage [W], xe2x80x9cXxe2x80x9d is a distance [mm] from one end of the thin-tube unit facing the discharge space to one end of the electrode coil facing the thin-tube unit, and xe2x80x9cLxe2x80x9d is a length [mm] of a part of the tubular member protruding from the end of the thin-tube unit facing the discharge space into the discharge space.
The above aim can also be achieved by a metal halide lamp, including: a bulb that is integrally made up of a light-emitting unit in which a discharge space is formed, and a pair of thin-tube units; a pair of electrodes that extend into the discharge space so that tops thereof are opposed to each other, each electrode having an electrode coil at a top part thereof; a pair of electrode holding members each being provided through a different one of the thin-tube units so that one end thereof holds the electrode and the other end thereof extends from one end of the thin-tube unit not facing the discharge space, each electrode holding member being sealed to the thin-tube unit via a sealing member; a tubular member that is provided in at least one of the-thin-tube units, so that the electrode and/or the electrode holding member therein is inserted through the tubular member, the tubular member being made of a heat-proof and heat-conductive material, wherein the expressions xe2x80x9cXxe2x89xa70.0056P+0.194xe2x80x9d and xe2x80x9c0 xe2x89xa6Lxe2x89xa60.44Xxe2x80x9d are satisfied, where xe2x80x9cPxe2x80x9d is a lamp wattage [W], xe2x80x9cXxe2x80x9d is a distance [mm] from a reference position to one end of the electrode coil facing the thin-tube unit, and xe2x80x9cLxe2x80x9d is a length [mm] of a part of the tubular member protruding from the reference position into the discharge space, the reference position being a position on a joining part of the light-emitting unit joining each thin-tube unit thereto, where an inner diameter of the joining part is 1.25 times an inner diameter of the thin-tube unit.
According to the above construction, when the metal halide lamp is lit with the electrodes in the arc tube being oriented in the vertical direction, the temperature of one ends of the thin-tube units facing the discharge space can be lowered to such a degree that an excess of light-emitting metal can exist in a liquid form. Also, the tubular member is embedded at a predetermined position in one or both of the thin-tube units so as to occupy its opening. Therefore, the light-emitting metal in a liquid form can be collected in such an area from which it is less likely to enter into the thin-tube unit Therefore, the amount of light-emitting metal flowing into the thin-tube unit can be reduced.
As a result of this, the metal halide lamp can maintain a sufficiently high vapor pressure within its discharge space and therefore can maintain stable characteristics with a reduced change in its color temperature even after continuous lighting of long hours.
Further, the tubular member is set in such a manner that the length of its part protruding into the discharge space does not exceed a predetermined value with respect to the distance xe2x80x9cXxe2x80x9d. Therefore, an arc discharge can be prevented from starting from the tubular member at the lamp startup.